In accelerating a particle beam a beam of particles passes through one or more cavities which are filled with radio frequency (rf) energy. The rf energy is transferred to the particle beam to accelerate the particles. When an accelerating particle beam enters a cavity that is filled with rf energy. a beam-loading effect transfers energy present in the rf field to the beam, causing th particles to accelerate. The beam-loading effect perturbs the amplitude and phase of the cavity rf field with resulting errors in the output beam energy.
Accordingly, rf control circuity is required to keep the phase and amplitude of the rf field constant while the cavity is being filled with rf energy when the beam enters the cavity, and while the beam is present in the cavity. Conventional rf field control is implemented with high speed analog circuitry. However, the limiting factor in the control bandwidth is the pole configuration of the cavity, which is not a static configuration because of thermal and mechanical changes to the cavity during operation of the accelerator. Satisfactory control is obtained during a steady state portion of the beam, but not during the field response from the initial beam transit of the cavity.
Feedforward systems have been used to reduce the errors in the output beam characteristics. However, conventional feedforward systems can only provide for a limited number of perturbing factors. In a typical accelerator rf drive circuit, several sources of loop perturbations exist, such as component nonlinearities, keystone anode voltage droop, and amplitude/phase cross coupling. Such perturbations are not constant but vary with time and component aging. Further, mechanical and thermal changes in the cavity structure cause the characteristic resonance of the cavity to vary. Thus, it is desired to provide an adaptive control loop to monitor the cavity rf field and to provide a time varying corrective signal to accommodate the initial portion of the beam transit through the cavity.
An adaptive feedforward control is provided in accordance with the present invention wherein a control function is successively modified until the cavity response is substantially constant for repetitive beam pulses.
Accordingly, it is an object of the present invention to proVide a feedforWard control loop which is adaptable to changing characteristics of the accelerator components by constantly monitoring the residual error signals.
It is another object of the present invention to provide a feedforward control system using digital signal processing technology.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.